WELLINGTON, New Zealand -- With up to 70 million possums munching trees and spreading bovine tuberculosis through New Zealand's farms, the country has no option but to continue using the pesticide 1080 to eradicate the problem, the nation's environmental control agency said Monday.

The Environmental Risk Management Authority said it reviewed the use of 1080 -- sodium fluoroacetate -- in New Zealand because of decades-old concerns that environmental damage from using the poison outweighs the benefits to the farming community.

The authority said it decided to allow the continued use of 1080, because without it possums would cost New Zealand farmers at least 5 billion New Zealand dollars (US$3.7 billion; euro2.7 billion) within 10 years by spreading bovine TB through cattle and deer herds.

Opponents of 1080 say the poison -- usually dropped from light aircraft -- can contaminate ground water and kill native birds, insects, frogs and other wild animals, as well as farm dogs.

"There is no practical alternative to the continued use of 1080 in areas where the preservation of our natural bush and agricultural production would otherwise be at serious risk," the authority's chairman, Neil Walter, said in a statement.

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"Pests like possums, rabbits, rats and stoats pose a major threat to New Zealand's environment and economy," he said. "On the other hand, 1080 is seen by many to pose unacceptable risks, particularly when it comes to aerial drops."

People planning to use 1080 must now be trained and licensed, and must notify all local residents that the poison is to be dropped in their area. The authority will monitor the impact of 1080 use and research alternative pest control, it said.

The decision to continue its use "does not give the aerial application of 1080 a green light so much as a flashing amber light -- proceed, but with caution," the agency said in its report.

The poison can be fatal for humans in large doses -- 1 milliliter diluted with 4 milliliters of water is enough to kill -- and there is no known antidote. But the environmental agency stressed that such quantities are rarely found in ground water and that the pesticide poses little threat to humans.

"Few harmful effects on human health have been identified from accidental exposure to 1080," the authority said, while urging those who use it to handle the poison with care.

New Zealand uses 80 percent of the 1080 pesticide produced worldwide, the environmental agency said.

Possums were brought to New Zealand from Australia for their fur, but a lack of natural predators means they now pose a major threat to the environment. Up to 70 million possums devour some 7 million tons (7.7 million short tons) of vegetation in New Zealand's forests each year.